The present invention relates to electrical filters, and more particularly to such filters made from piezoelectric materials.
In one form of filter, a disc-like ceramic piezo-electric resonator, having opposed metallic surfaces thereon, is suspended inside a casing. The suspension is often by wires or springs which also provide the current leads to and from the resonator.
In one form of prior device, the lead wires were soldered to the metallic outer surfaces of the resonator. Such approach had the disadvantage that the device needed to be tuned after the soldering operation was completed. Such tuning not only required an extra step, but the step was often difficult, because it required that a precise amount of ceramic material be uniformly removed from the circumference of the resonator. Moreover, the soldering of the wires established a type of mechanical connection which added spurious resonances which interfered with the operation of the resonator.
Another form of prior device used a spring type of mounting for the resonator. In such mounting electrically conductive springs directly contacted the generally planar faces of the resonator. Such approach was developed to eliminate the difficulties resulting from soldering wires to the metallic surfaces of the electrode. However, the pressure contact required precise control of the internal dimension of the case or housing to hold the movement of the resonator, as a result of shock or vibration, to a tolerable minimum. In view of the practical requirements of an unrestrained circumference, and the dimensional variations of the case and resonator, a small amount of movement occurred at the point of contact and support. Such movement resulted in a small shift in resonant frequency and a variation in mechanical Q, which are undesirable effects. Such movement also caused a wearing of the silver electrodes, which sometimes led to intermittant operation.
Moreover, special problems existed when two or more resonators were mechanically coupled together to form multiple resonator designs, such as, for example a two resonator three-terminal design. Such multiple resonators were costly, difficult to fabricate and assemble and generally provided poor control of filter center frequency and band width.
One form of prior multiple resonator device used a coupling rod passing through the machined out center of adjacent discs, and another machined a monolithic structure, of resonators and integral connecting rods, from a one-piece solid structure. In neither device was there a projection from the outside face of the end resonators. Such structures provided end resonators loaded mechanically on one face only (the internal faces). The effect was to increase the number of spurious resonances, as well as to place restrictions on the maximum diameter of the coupling rod. As a result, if the coupling rod was reduced in diameter to reduce spurious resonances, the device was not as strong as desired and subject to damage from vibration and shock.
As an attempt to eliminate the problems of the coupling rod type of device, solid extensions were fabricated on one face of individual resonators at the node of vibration. The solid extensions were then coupled to each other by a conducting epoxy which fastened the extensions together as described in U.S. Pat. No. 3,051,919. However, with extensions or stems on only one side of each resonator, a structure resulted having the aforesaid problems. In addition, such coupling resulted in poor filter performance under certain conditions of time and temperature, as the elastic properties of the epoxy changed with time and temperature. The device was also fragile.